Numerous VPO catalysts (sometimes referred to as mixed oxides of vanadium and phosphorus), substantially in the form of vanadyl pyrophosphate, optionally containing a promoter component, have been previously disclosed as being useful for the conversion of various hydrocarbon feed stocks to maleic anhydride. In general, such catalysts wherein the valence of the vanadium is less than +5, usually between about +3.8 and about +4.5, are considered particularly well suited for the production of maleic anhydride from hydrocarbons having at least four carbon atoms in a straight chain or cyclic structure. Common organic feed stocks include various hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, benzene or mixtures thereof.
VPO catalysts are usually prepared by contacting vanadium-containing compounds, phosphorus-containing compounds, and promoter component-containing compounds (when a promoter element is desired) under certain conditions sufficient to reduce pentavalent vanadium to the tetravalent state and form the desired catalyst precursor comprising vanadyl hydrogen phosphate, optionally containing a promoter component. The catalyst precursor is recovered by separation such as filtration and typically in particulate form having particle size ranging from few microns to hundred microns. Then the precursor is formed into a certain shaped body such as tablet or pellet by compression. Typically, a lubricant such as graphite or boron nitrate is blended into the precursor composition before compression to facilitate the tableting or pelletizing process. Finally the shaped body undergoes a step called activation, which is carried out under certain atmosphere and temperature program, to transfer the catalyst precursor into an active catalytic component.
In its final form, the catalyst comprises a mass of porous tablets or pellets which are charged in bulk to provide the catalyst bed of a fixed bed reactor. Typically, the catalyst is charged to a tubular reactor comprising the tubes of a shell and tube heat exchanger. Hydrocarbon(s) and oxygen are fed to the tubes, and a heat transfer fluid, such as molten salt, is circulated through the shell to remove the exothermic heat of the oxidation reaction.
The porous nature of the catalyst contributes substantially to the active surface area at which the catalytic reaction takes place. For the internal surfaces of the catalyst body (tablets or pellets) to be utilized effectively, the feed gases, hydrocarbon and oxygen, must diffuse through the pores to reach the internal surfaces, and the reaction products must diffuse away from those surfaces and out of the catalyst body.
It is known in the art that resistance to internal diffusion in the catalyst bodies can become a rate limiting factor in the reaction. One method used to overcome resistance to internal is to shorten the diffusion paths by using relatively small catalyst granules. However, small catalyst granules will results in increased pressure drop through the fixed bed, leading to operational difficulties.
Another method used to increase catalytic performance has been to focus on increasing and modifying the catalyst's macro-pore structure. As mentioned above, during the preparation of VPO catalyst, catalyst precursor is compressed into various shapes of tablets or pellets. Inside of the tablets or pellets, pores are formed. There are some micro-pores having pore diameter ranging from 0.01 to, and including, 0.6 microns. These micro-pores are inherently formed among catalyst precursor particles by packing these precursor particles together. Macro-pores, however, are pores with a pore diameter ranging from above 0.6 to about 10 microns that are not inherently formed, rather are formed using pore building agents. Mount et al. in U.S. Pat. No. 4,092,269 disclose that a desired fraction of macro-pores having a size of 0.8 to 10 microns are prepared by adding a pore modification agent to the precursor at any stage prior to activation. After precursor with pore modification agent was compressed into a certain shape, the pore modification agent is removed to generate the macro-pores. Typically, to remove the pore modification agent, calcination of the precursor is common method conducted at a temperature between about 300° C. and 600° C. U.S. Pat. Nos. 4,699,985; 5,773,382 and 5,275,996 also describe the preparation of a maleic anhydride catalyst using pore building agent to generate macro-pore structure.
Increasing pore volume by adding macro-pores is reported to increase catalytic activity. For example, Zazhigalov, et al, “Effect of the Pore Structure and Granule Shape of V-P-O Catalyst on the Selectivity of Oxidation of n-Butane,” Zhurnal Prikladnoi Kimii, Vol. 61, No. 1, pp. 101-105 (January, 1988) report that the activity of VPO catalysts in the oxidation of n-butane increases with an increase in the total pore volume and the introduction of macro-pore having pore diameter bigger than 0.8 microns. However, greater efficiency continues to be desired for these catalysts.